DDS should be cost-effective. In devising a cost-effective service, all types of costs, viz, direct as well as indirect costs should be taken into consideration. Direct cost is the cost of operating the service i.e., cost Of processing of requests, copying the document, postage, etc., whereas indirect cost includes the cost of building collection, salaries of the staff, cost of equipment, etc. Service is more cost-effective if it is offered from a centralized collection and the' number of requests is large. Conversely, it is less "cost-effective if it is based on decentralised collection. It is increasingly realised that the service can be more cost-effective if it is operated by building a core collection to meet primary needs of the users and for residual requests, accessing speedily the material from external sources.